Re: [PATCH 3/8] Use process freezer for cpu-hotplug

[Nathan Lynch <ntl@pobox.com>]

Ingo Molnar wrote:
> 
> * Nathan Lynch <ntl@pobox.com> wrote:
> 
> > > -	raw_notifier_call_chain(&cpu_chain, CPU_LOCK_ACQUIRE, hcpu);
> > > +	if (freeze_processes(FE_HOTPLUG_CPU)) {
> > > +		thaw_processes(FE_HOTPLUG_CPU);
> > > +		return -EBUSY;
> > > +	}
> > > +
> > 
> > If I'm understanding correctly, this will cause
> > 
> > # echo 0 > /sys/devices/system/cpu/cpuX/online
> > 
> > to sometimes fail, and userspace is expected to try again?  This will 
> > break existing applications.
> > 
> > Perhaps drivers/base/cpu.c:store_online should retry as long as 
> > cpu_up/down return -EBUSY.  That would avoid a userspace-visible 
> > interface change.
> 
> yeah. I'd even suggest a freeze_processes_nofail() API instead, that 
> does this internally, without burdening the callsites. (and once the 
> freezer becomes complete then freeze_processes_nofail() == 
> freeze_processes())

Yeah, I just realized that an implementation of my proposal would busy
loop in the kernel forever if a silly admin tried to offline the last
cpu (we're already using -EBUSY for that case), so
freeze_processes_nofail is a better idea :-)

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